1. Field of the Invention
This invention relates, in general, to hard copy printers and, more specifically, to electrophotographic copiers, duplicators, and printers suitable for copying multi-page documents.
2. Description of the Prior Art
Devices which produce hard paper copies of text, graphics, and/or pictorial images are generally referred to a copies, duplicators, or printers. Such devices, referred to herein generally as printers, are often required to print several copies of a multi-page document. In order to print several copies of a multi-page document the image on each page must be transferred to the printer several times. This requirement often means that the complete stack of pages must be scanned once for each copy desired.
One method used in the prior art to print multi-page documents is to recirculate the originals through an optical scanner each time the page is to be printed. While this arrangement has been successful in the past, two primary disadvantages to such a system exist. First, mechanically transferring the originals through a scanner many times can cause deterioration of the original and increase the probability of the paper path being jammed. Second, the time it takes to physically move the originals through the image scanner is becoming a limiting factor in the through-put or overall speed of the printer.
Some prior art printers use electronic scanning to solve these problems. With electronic scanning, a digitized image of each page is stored in a digital memory bank. When each page is to be printed, the printer's controller obtains the digital image from memory rather then directly from the original. The main drawback to the memory system is the size of the memory required for large documents. Digitized information requires considerable memory space and efforts to compress digital image data have not fully solved the massive memory requirements.
It is not uncommon that the printer must have the capability of making several copies of a 50-page document. Compression techniques applied to printers work satisfactorily for text and most graphics, giving a compression ratio of about 30 to 1. Pictorial images, or pictures, generally require different, more complex, compression techniques than those used for text and graphics. Often, pictures require less memory space when no compression is attempted. Assuming 50 pages with 20 percent picture content, a resolution of 400 dpi (dots per inch), and 81/2 by 11 inch paper, approximately 21 Mbyte of memory would be required to store the digital images for all 50 pages. This approximation does not consider the possibility that much more memory would be needed if the originals contained more than 20 percent pictorial information.
Solid state memory, such as DRAM and CMOS RAM, is relatively expensive in such quantities. Winchester hard disk drives capable of the required bit transfer rate are also expensive. Streaming, or conventional linear motion, tape systems may be cost effective, but traditionally are too slow for this application. Even considering the availability of streaming tape systems having sufficient speed to record and playback the digital data, there is a further problem of the time necessary to rewind the recorded tape to start reading the data for another set of printed pages. Since the streaming tape system operates with a fast tape speed during recording to achieve a high bit rate, a considerable length of tape must be rewound to obtain the data for another set of prints. Typically, several seconds would be required and such an amount of time significantly detracts from the overall through-put of the printing system.
Therefore, it is desirable, and it is an object of this invention to provide a magnetic storage system for a multi-page printer which can handle the data rates necessary for good reproduction of the original. It is also desirable, and it is another object of this invention, to provide a storage system wherein the time delay caused by tape rewind is considerably reduced.